Because he’s using a non-slant-parallelogram, 1x-specific SRAM rear derailleur on what’s effectively a 2x setup.
<gear nerd>
The slant-parallelogram derailleurs that we’ve all been riding since Suntour’s patent expired in '84 are designed such that the angle of the parallelogram is what keeps the guide pulley tracking close to the cassette cogs through the entire gear range. The guide pulley is very close to the cage pivot (and on some newer designs, actually concentric with the cage pivot), so that the pulley position doesn’t really change when you shift into a different chainring and the cage pivots to a new position. This allows the derailleur to properly operate in all of the cogs in each of the chainrings.
(Aside: The angle of the parallelogram is the main defining difference between traditional “road” and “mountain” derailleurs. You could put an extra-long cage on a road derailleur body, and the guide pulley would just run into the big cassette cogs. Similarly, put a short cage on a mountain derailleur body, and the guide pulley will be floating in space way away from the cassette in the 25T “big” cog.)
On SRAM’s 1x system (and the new Shimano XTR 9100 1x derailleurs), the parallelogram isn’t slanted anymore. Rather, the guide pulley is highly offset from the cage pivot - so the rotation of the pulley cage is actually what re-positions the guide pulley into the correct location for each cog. It should be obvious why this won’t work with multiple different-sized chainrings.
So that’s why he’s twiddling the B-screw when he shifts with a stick.
(But for fuck’s sake, he’s friction shifting the rear already. He should just throw a Shimano slant-parallelogram derailleur on there; I’m sure an 11-speed XT long cage would work with 12-speed chain and a SRAM 10-50 cassette, and then he wouldn’t have to twiddle with the derailleur every time he wants to be in the other ring)
</gear nerd>